Shear deformability characteristics of a rapid-cure woven prepreg fabric

One of the materials used for this study is a vinyl-based thermoset prepreg particularly developed to manufacturing press cured structural parts for the serial automotive industry. The second material considered is the dry fabric used in the same prepreg. The materials were provided by Solvay Heanor UK. Further details of the materials are as follows:

Figure 1 shows images of both the prepreg and dry fabric in the non-deformed state. The selected prepreg is ideally suited for high-rate manufacture of components using compression molding (press curing) process. The prepreg can be used to manufacture components with a total cycle time of 3 min using a recommended cure temperature of 150 °C within a pre-heated compression molding tools.

It is to be noted that the constituent dry reinforcement fabric of the prepreg material investigated here does not contain any binder material.

Figure 2 shows the viscosity of the vinyl-based resin in the prepreg material at different temperature. The viscosity vs temperature curve is plotted as a log-linear graph to demonstrate the change in viscosity at a relevant range of temperatures.

The bias extension test is a de facto standard method to measure the in-plane intraply shear behavior for the fabrics and prepregs. The testing needs a careful cutting of the specimens, clamping in the specifically designed grips, selection of suitable test conditions preferably relevant to industrial process applications. The material geometry is of a rectangular shape where length of the specimen is typically double or more than its width to obtain a suitable state of shear i.e. three zones of ‘pure shear’, ‘half shear’ and ‘undeformed’ as shown in Fig. 3. The orientation of the fibers is maintained at ±45° to the loading direction in the undeformed configuration. The overall size of the bias extension specimen in these particular tests is 250 mm in length and 74 mm in width (effective gauge dimensions of ‘220 mm × 74 mm’). Figure 3 shows the images of the bias extension specimens for the prepreg and dry fabric, in their undeformed and deformed configurations. In the bias extension test, the overall deformation of the specimens are shown to be equivalent in terms of undeformed, half-shear and pure-shear zones for the prepreg and its dry fabric.

The shear angle γ (radians) is measured theoretically in the pure shear zone of the bias extension specimen [11] as:

Here δ is the change in length of the specimen and L = H – W; where H and W are the original height and width of the specimen, respectively.

The applied force data from the bias extension tests can be converted into normalized shear force using an iterated process indicated in [11] as:where F_sh is the normalized value of the shear force (in N/mm).

Eq. (2) is a relationship between applied force, normalized shear force and shear angle in the pure and half shear zones of the bias extension specimen.

Intraply shear behavior of prepregs as well as of dry fabrics with biaxial fibrous reinforcements can be characterized using picture frame tests. In the picture frame test, the sample is initially a cruciform-shaped. The specimen is held in the test-rig where the tows are aligned with the outer edges of the rig or oriented in the ±45° position to the loading direction. Figure 4 shows the undeformed and deformed geometries of the prepreg material. The picture frame test creates uniform shear for the entire specimen under investigation. Because the test is very sensitive to any misalignments in the orientation of fibers, extreme care is required when cutting the specimen as well as clamping it in the test-rig.

The effective specimen size used for the picture frame tests is ‘134 mm × 134 mm’. The picture frame test-specimens were prepared using a Zund CNC cutter where fiber alignment errors arising due to manual cutting were reduced. The shear angle, γ, can be calculated from the geometry of the picture frame as:

Where L_frame is the side length of the picture frame jig between hinges, as indicated in Fig. 4 and d is the extension applied on the frame.

The force data of picture frame tests can be normalized [11] using the relationship:

Where F_normalized is the force normalized using an energy method, and F_s is the shear force obtained such that:where 2θ is the angle of the frame (Fig. 4) and \( \theta =\frac{\pi }{4}-\frac{\gamma }{2} \). The effective fabric length used for investigations is denoted by L_fabric.

Since the picture frame test provides a uniform shear strain field in the specimen, it is possible to interpret the force-displacement data in terms of shear stress versus shear strain. This provides an easy means to obtain the shear rigidity of the material [25]. The shear rigidity computed in the form of a polynomial expression is useful in order to numerically analyze the formability of composite plies of either a prepreg or a dry fabric [25‐28].

The intraply tests were performed on Instron-5800R tensile and compression testing machine equipped using an environmental chamber with a load-cell capacity of 1 kN. For each test type and unique parameter set, at least three identical test repeats were carried out (depending on qualification of test concerning the level of variability) and the graphs are plotted as average of the test data considered qualified.

Table 1 summarizes the test conditions used for both the picture frame and bias extension methods using a range of temperature and shear rates. Both tests for the prepreg material are performed with equivalent shear rates of 0.94° s⁻¹ and 4.9° s⁻¹ at different temperature conditions of temperature (i.e. RT (23 °C), 50 °C and 80 °C). In contrast, the dry fabric is characterized using a single shear rate of 0.94° s⁻¹ (an axial stretch rate of 1 mm/s in the case of bias extension test) at room temperature only. This is because the dry fabric material behavior is shear rate and temperature independent, especially when there is no binder involved.

Correct measurement of the intraply shear properties of composite reinforcements and prepregs is linked to the actual shear angle that develops during in-plane shear deformation of materials using the picture frame and bias extension tests. It has been shown by Pasco et al. [10] in the case of prepregs and Zhu et al. [13] in the case of dry fabrics that theoretical measurement of shear angle in bias extension tests based on PJN approach deviates from the actual shear angles after a certain stage of shear deformation. It is therefore important to employ a technique that correctly measures the actual shear angle, specifically during the bias extension tests in the so-called pure-shear zone. In contrast, the picture frame test does not require external shear angle measurement, because an overall uniform shear deformation exists within the acceptable limits over the entire specimen [10, 16, 17]. As a consequence, the determination of the material shear angle can be done using the rigid frame angle. For the bias extension tests in the current investigations, a video extensometer was used to track the axial and transverse displacements of the dots printed on the warp and weft tows of the specimen in the pure shear zone as shown in Fig. 5.

The actual shear angle can be obtained using a simple trigonometric relationship:

With: γ is the measured shear angle, a is the axial strain length and b refers to the transverse strain length as shown in Fig. 5.

Because both the bias extension and picture frame tests are supposed to be used alternatively for measuring the shear behavior of biaxial reinforced materials, the data generated from two tests should be comparable. This necessitates using similar conditions of temperature and testing speed in the two test methods due to the temperature and shear-rate dependent viscosity of the prepreg material. Based on a recent test campaign by the authors presented in [10], the test speed has been converted into shear rate. Initially the shear rate obtained during a bias extension test was determined from the measured shear angle value and then based on this shear rate, the required crosshead speed for the picture frame test was calculated. Fig. 6 shows the measured shear angle values (in the pure shear zone) plotted against time for a bias extension test performed with a crosshead speed of 1 mm/s. The shear angle varies in a quasi-linear trend with time, indicating a constant shear rate within the reinforcement during the test. The slope of the linear fit corresponds to the value of the shear rate in degrees per second (° s⁻¹). In this case, a bias extension test performed with a crosshead speed of 1 mm/s produces a shear rate within the material of approximately 0.94° s⁻¹.

Based on this value, it is then possible to calculate the required crosshead speed needed during a picture frame test to produce a similar shear rate. It is shown in [29] that the angular shear rate during a picture frame test can be expressed as:

Where \( \dot{\gamma} \) is the shear rate, \( \dot{d} \) and d are the crosshead speed and displacement, respectively, and L_pf is the side length of the frame. Rearranging Eq. (6), the required crosshead speed for a given angular shear rate can be expressed for any displacement of the picture frame such that:

Figure 7 shows the required relationship between the crosshead speed and the crosshead displacement in order to obtain a constant shear rate of 0.94° s⁻¹ during a picture frame test. It can be seen that this relationship is not linear, and slower crosshead speed are required as the test progresses in order to maintain a constant shear rate. This relationship was then programmed in the universal tensile testing machine, as a test speed based on an equation was not made possible using the machine software. Therefore, the function was discretized in small discrete steps of constant speed. The aforementioned procedure was then used in the case of bias extension test for an axial stretch rate of 5 mm/s to get an equivalent shear rate of approximately 4.9° s⁻¹ for the picture frame tests.

The bias extension test results are presented as overall applied force (N) versus shear angle (°) measured in the pure shear zone of the specimen for the woven prepreg and fabric (Fig. 8). The error bars represent one standard deviation on either side of the mean. Figure 8 shows the test results for the prepreg and its constituent dry fabric at different temperatures and strain rates. The results are presented in the log-linear form of force and shear angle to better approximate the magnitude of force at each stage of deformation.

The picture frame tests were also performed for both the prepreg and its constituent dry fabric. The test conditions used in picture frame tests were kept similar to the corresponding bias extension tests to compare the results from two methods. It is to be noted that the overall number of picture frame tests performed are more than double the number of tests considered qualified. The tests with lower level of applied force were considered qualified, as indicated in [10]. This is because the misalignment of tows generates spurious tensions in the fibers that ultimately influences the actual shear loads during characterization of the material [15].

Figure 11 shows the results of the influence of temperature and strain rate on the prepreg fabric with the picture frame testing. The results show a similar intraply shear behavior as it was observed in the bias extension testing. Overall, there is a continuous drop in the applied force with the increase of prepreg temperature (Fig. 11). It can also be observed that increase of shear rate generates higher loads in the case of prepreg testing for all selected states of temperatures. As mentioned in Section 2.3, the picture frame test typically produces inconsistent test results being highly sensitive to misalignment and pre-stressing issues. In this context and to reduce the variability in the current testing campaign, the picture frame results with higher shear force were eliminated and the tests with lower force were considered qualified. Specifically, Fig. 12 shows the difference in level of the applied force observed at a shear angle of 40° for different temperatures and strain rates. At a shear rate of 0.94° s⁻¹, the difference in the force is quite insignificant for the elevated temperature testing of 50 °C and 80 °C. However, the difference in the magnitude of force to shear deform the prepreg from RT to 50 °C is quite considerable, i.e. 320 N to 3.7 N respectively, at a shear angle of 40° for the case of shear rate tests at 0.94° s⁻¹ (Fig. 12).

According to Fig. 12, the prepreg fabric results of intraply shear at 80 °C are of the similar order of magnitude to that of the dry fabric. Again, this indicates the shear deformation behavior of the prepreg fabric at elevated temperatures (~80 °C) is equivalent to its dry fabric reinforcement. The applied force is quite comparable between the dry fabric and prepreg at a shear angle of 40°, irrespective of the shear rate influence at 80 °C. Again, these observations suggest that intraply shear dependent formability characteristics of a prepreg fabric converges to that of its corresponding dry constituent fabric at elevated temperatures.

As indicated in Section 2, the two tests of picture frame and bias extension are de facto standard tests for measuring intraply shear properties of biaxial reinforced materials, therefore, the results obtained from two tests should produce the same output ideally. Although the main difference is in the end-conditions of the tows of the specimen, yet it is worth comparing the results of the two tests for the same material, using Eqs. (2) and (4), to draw important conclusions on the suitability of the tests for any particular material. Further, the test conditions of shear rate and temperature should be same for both tests, especially when testing prepreg materials. It should noted that the normalization exercise is limited to the two test cases (i.e. for the prepreg using 50 °C at 4.9° s⁻¹; and for the dry fabric at 0.94° s⁻¹).

Figure 13 shows a comparison of the intraply shear behavior of the prepreg determined after normalization of the data from the two tests. The data is presented in the form of normalized shear force (N/mm) vs measured shear angle (°) at 50 °C using an equivalent shear rate of 4.9° s⁻¹ for the bias extension and picture frame tests. The normalized shear force obtained from both tests coincide very well throughout. However, the normalized shear force obtained from the two test methods in the case of dry fabric tends to deviate from one another as shown in Fig. 14.

This deviation can either be attributed to the difference in the boundary conditions of two test methods or due to the poor material integrity of the dry fabric and the lack of viscous resin. Based on the correlation of prepreg results presented in Fig. 13, it can be argued that the deviation in the results in Fig. 14 is mainly dependent on the lack of integrity of the dry fabric tows (due to lack of any viscous resin) at the crossover points. Further, in the case of bias extension tests, the insufficient integrity of the material and free end conditions of the tows fail to provide the required kinematics. This results in significant inter-tow slippage that was also evident from the actual measurement of shear angles shown in Fig. 10. In contrast, the picture frame tests provide homogeneous deformation state due to rigid clamping of the specimen at the tow-ends. These observations suggest that, when testing material with a relatively poor integrity (as is the case for dry fabric and a prepreg at elevated temperature), the picture frame method might be more appropriate for correct measurement of the intraply shear properties.